FIELD OF THE INVENTION
The invention relates to atomizing furnaces for spectroscopy and, more specifically, to such an atomizing furnace formed of carbon material which is electrically transversely or longitudinally heated. The furnace is assembled from a tube furnace segment formed with a sample insertion opening and a separately produced specimen support. The tube furnace segment, in whose interior chamber the atomization takes place and which has on the outside contact elements for connection for electrical heating, is formed with a recess for receiving a peg located on the underside of the specimen support. The recess is formed in the inside wall at the side lying approximately opposite to the sample insertion opening. The specimen support, which serves for the uptake and delayed vaporization of the sample to be analyzed, is disposed and held in the inside wall of the tube furnace essentially outside the path of the working beam. The specimen support is held in the tube furnace segment by a peg that extends from its outside wall downwardly to the wall of the tube furnace segment and the specimen support being held by means of this by insertion in a recess in the wall of the tube furnace segment corresponding to the shape of the peg. Atomizing furnaces of this type are preferably used for flameless atomic absorption spectrometry on the basis of graphite tube technology (GF-AAS) for the vaporization and the atomization of solid and liquid samples.
It is an essential object in GF-AAS to delay the thermal atomization of the sample with respect to the heating of the inner chamber of the atomizing furnace. This ensures that the constituents of the sample vaporize under approximately stabilized temperature conditions and are atomized suddenly and cannot precipitate on comparatively cooler parts of the walls of the inner atomization chamber. This object is satisfied in the prior art disposing a specimen support in the inner furnace chamber. In the ideal case, the specimen support should for this purpose be constructed and fixed in the furnace in such a way that it is heated neither by heat conduction nor by Joulean heat, but instead exclusively by radiant heat from the inside wall of the furnace. An configuration with a longitudinally heated atomizing furnace with a specimen support--which configuration, however, fulfilled the above-mentioned requirements to only a certain extent--was first suggested by L'vov, 33B Spectrochimica Acta, pp 153-93; Pergamon Press, U.K. 1978.
Further developments of specimen supports for longitudinally heated atomizing furnaces appeared in various patent publications, including German patent DE 29 24 123, German utility models GM 87 14 926.5, GM 88 03 144.6, German published, non-prosecuted applications 37 22 379 and 38 23 346.0, and in European published application EP 0 442 009. All of those teachings, however, are still subject to the essential disadvantages with regard to the above-mentioned requirements.
A further, improved specimen support for a longitudinally heated atomizing furnace is described in the East German publication DD 233 190 A (West German publication number 35 45 635). The support is point-fixed by way of a pin-like support disposed asymmetrically with respect to the tube furnace center and is inserted into an indentation formed in the inside wall of the tube furnace. The specimen support can, however, be removed from the tube furnace at any time. One of the express objects of this disclosure was that atomizing furnace and specimen support do not form a non detachable unit in the operative final state of production, because the specimen support itself can be inserted and removed by a manipulator. The result is that the position of the specimen support in the atomizing furnace is not positively fixed particularly in the case of shaking or in the presence of strong magnetic fields. Tests by the applicant resulted in uncontrolled wall contacts of the specimen support and thus current conduction and heat conduction between the outside edges of the specimen support and the inside wall of the furnace. This consequently led to irreproducible relationships from measurement to measurement. The specimen support is designed to receive only small volumes of substance to be analyzed (&lt;10 .mu.l) and is to be produced only from vitreous carbon or pyrocarbon. Vitreous carbon and solid pyrolytic carbon can be used as materials for specimen supports to only a limited degree, because the analytical determination of refractory-carbide-forming GF-AAS substances for analysis from surfaces of this type is not possible, the required material purities can be realized only with difficulty, and the cost-performance ratio is unfavorable for the user.
Transversely heated atomizing furnaces were first introduced in 1987 (cf. German utility model GM 87 14 670). European patent publication EP 0 321 879 A2 (corresponding to U.S. Pat. Nos. 4,968,141 and 4,961,645) describes an atomizing furnace with a specimen support in a longitudinally heated embodiment and in a transversely heated embodiment, which specimen support is connected to the inside wall of the furnace in a non detachable manner by way of a web which lies symmetrically with respect to the center of the furnace.
Specimen support and furnace form a material structural unit, which is produced from one crude body.
The specimen-holder portion extends only over a central region of the furnace portion. Consequently, there is only a small receiving volume for the substance to be analyzed. The connecting web itself has a plurality of transverse bores as a material-reducing measure. An atomizing furnace of this type, consisting of a solid graphite blank can be produced only at great technical expenditure. This obviously has a negative effect on the price to the user for this wear part.
Specimen supports with supporting rings for transversely heated atomizing furnaces in accordance with German Patent 42 43 767 C2 can likewise be produced only at high cost and with great technical expense, although both the specimen support and the furnace can each be produced as a component part.